1. Field of the Invention
The present invention relates to organic electroluminescence (EL) elements, organic EL display apparatuses using the organic EL elements, and a manufacturing method of the organic EL elements.
2. Description of the Related Art
Light-emitting devices using organic EL elements are expected to be applied as next-generation displays which replace cathode ray tubes (CRTs), liquid crystal displays (LCDs), and plasma display panels (PDPs) as thin and lightweight light-emitting devices of a self-luminous type.
Organic EL elements are classified by its manufacturing process into a coating type and a deposition type. In fabricating organic EL elements of the coating type, an inkjet process is currently predominant. In applying a luminescent material in this inkjet process, the luminescent material needs to be printed on only a specified area of pixels, but with only discharge accuracy in the inkjet process, it is difficult to achieve coating with required accuracy for such limitation of the coating area.
An existing solution to the above difficulty is to form bank-like shapes called banks on pixel electrodes, and a luminescent material is discharged and applied to recesses in the middle of the banks (Japanese Unexamined Patent Publication Application No. 2004-127551).
In most cases, a photosensitive resin is used in the process of forming banks. Specifically, in many cases, the banks are formed by exposing, via a mask of a predetermined pattern, a substrate coated with positive or negative photosensitive resin, to light from a light source which emits i line, g line, or h line, or emits all of these lines, followed by development and other processes. Thus, a method of forming banks using a light source which emits light of short wavelengths is predominant.
In many cases, at the time of providing an actual substrate with a layer of photosensitive resin which will be banks, there is already a structure in which a transparent electrode, a reflecting electrode, a hole transport layer (hereinafter referred to as HTL), or the like is stacked on the substrate. Now, a problem is described which occurs in an organic electroluminescence element of so-called a top emission type (hereinafter referred to as organic EL element) that uses a metal having relatively high reflectance for the reflecting electrode.
In some cases of an organic EL element of the top emission type, banks are formed on a substrate on which, for example, a 100 nm-thick molybdenum chrome electrode (hereinafter referred to as MoCr electrode) is formed as a reflecting electrode and a 40 nm-thick indium tin oxide electrode (hereinafter referred to as ITO electrode) is further stacked.
In such cases, a so-called cavity effect between the bank layer and the set of MoCr electrode and the ITO electrode results in dependence of reflectance on wavelengths. The dependence of reflectance on wavelengths is based on a so-called optical resonance effect indicating that when an optical path length between the respective layers is equal to the integral multiple of a certain wavelength, the light of that wavelength is reflected on the MoCr electrode and emitted with high intensity.
For example, the optical path length changes according to an amount of changes in thickness of the ITO electrode, leading to drastic changes in intensity of the reflection. Especially, the reflection intensity of light for exposure having a short wavelength in the ultraviolet light range, etc., changes with high sensitivity depending on changes in the thickness of the ITO electrode.
The photosensitive resin for the banks is influenced by the light reflected from the base at the time of exposure. For example, in a positive photosensitive resin, as the intensity of the light reflected from the base changes, acid generation by the exposure changes, with the result that the shapes of the banks after the development become different. Furthermore, also in a negative photosensitive resin, as the intensity of the light reflected from the base changes, a degree of polymerization, a degree of cross-linkage, and the like change, with the result that the shapes of the banks after the development become different.
In particular, in an organic EL display apparatus including a plurality of organic EL elements arranged in a plane, in-plane variations in thickness of the ITO electrode make the intensity of the light reflected from the base different in the plane. This deteriorates in-plane uniformity of shape (including a thickness and a taper angle) of the formed banks, resulting in deteriorated in-plane shape uniformity of a luminescent material layer that is formed in the inkjet process.
As above, because the shape of the formed banks is influenced by the thickness of the ITO electrode, not only it is difficult to fabricate an organic EL element having a desired bank shape, but also there is an unfavorable problem of in-plane unevenness (so-called luminance variations) in luminescence characteristics of the organic EL display apparatus.
The present invention has been devised in view of the above conventional problems, and an object of the present invention is to provide an organic EL element capable of reducing errors in shape of formed banks and an organic EL display apparatus which uses such organic EL elements and has excellent in-plane uniformity of luminescence characteristics.